1. Field of the Invention
The present invention relates to a color registration method and a color image forming apparatus.
2. Description of the Related Art
There has been known a color image forming apparatus (so-called tandem-type color image forming apparatus) having a plurality of drum-type photoconductors. In the color image forming apparatus, it is important to suppress the positional deviation for every color (color misregistration) to an unnoticeable degree. When the color misregistration is great, it might be evaluated that the image quality is deteriorated. The greatest factor of the color misregistration is a periodic crude density on the output image caused by the eccentricity of each photoconductor. The ideal countermeasure is that the eccentric amount of each photoconductor is sufficiently reduced, but trade-off between cost and mass-productivity should be considered.
In view of this, various ideas have been provided to make the color misregistration unnoticeable even if the eccentric amount is the same. For example, an apparatus in which the peripheral length of each photoconductor drum and the peripheral length of the transfer belt is set to have ratios of whole numbers has been proposed (for example, Japanese Patent Laid-Open No. 7-261499). Further, an apparatus has been known in which a mark formed on a transfer belt is detected and, when the detected position is outside the fixed range, this position is stored and the formation of the mark is inhibited at this position (for example, Japanese Patent Laid-Open No. 2004-294471).
When the phase (rotational phase) of the eccentricity of each photoconductor is not matched, the color misregistration becomes noticeable. This point is focused, and various ideas have been given for matching the rotational phase of each photoconductor on the output image so as to make the color misregistration unnoticeable. In this case, in order to detect the rotational phase of each photoconductor, a toner pattern (toner image) having lines, parallel to the rotational axis of the photoconductor, arranged at equal intervals in the rotating direction is formed, and the deviation from the expected position is detected.
However, the cause of the crude density of the output image, in other words, the cause of the deviation of the formed toner pattern from the expected position, exists in the factors other than the eccentricity of the photoconductor. Upon detecting the rotational phase, the causes other than the eccentricity of the photoconductor become a disturbance. A sufficient precision cannot always be obtained in the detection of the rotational phase of each photoconductor using the toner pattern due to the disturbances.
In order to precisely detect the rotational phase of each photoconductor, a technique for effectively removing the disturbance component has been desired.
The color image forming apparatus performs the image formation by using three primary colors of yellow, cyan, and magenta, and black. The tandem-type image forming apparatus includes four photoconductors corresponding to each color. In the case of the monochromatic image formation, only the black photoconductor is used.
It is preferable that the diameter of the black photoconductor is increased from the viewpoint of achieving high-speed monochromatic image formation, compared to the color image formation, and of prolonging the service life of the black photoconductor to make its exchange cycle same as that of the other photoconductors. However, if only the diameter of the black photoconductor is greater than the diameter of the other photoconductors, various subjects involved with the color image formation arise. The representative one is the subject relating to the color misregistration. Since the rotational cycle of the black photoconductor is different from those of the other photoconductors, the technique for aligning the direction of the eccentricity to make the color misregistration unnoticeable cannot be taken.
A technique for making the color misregistration unnoticeable with a simple configuration has been desired even in case where a plurality of types of photoconductors, each type having a different diameter, are used.